Method of fabrication of a torsional micro-mechanical mirror system

ABSTRACT

A torsional micro-mechanical mirror system includes a mirror assembly rotatably supported by a torsional mirror support assembly for rotational movement over and within a cavity in a base. The cavity is sized sufficiently to allow unimpeded rotation of the mirror assembly. The mirror assembly includes a support structure for supporting a reflective layer. The support structure is coplanar with and formed from the same wafer as the base. The torsional mirror support assembly includes at least one torsion spring formed of an electroplated metal. An actuator assembly is operative to apply a driving force to torsionally drive the torsional mirror support assembly, whereby torsional motion of the torsional mirror support assembly causes rotational motion of the mirror assembly. In another embodiment, a magnetic actuator assembly is provided to drive the mirror assembly. Other actuator assemblies are operative to push on the mirror assembly or provide electrodes spaced across the gap between the mirror assembly and the base. A process for fabricating the torsional micro-mirror is provided. The torsional micro-mirror is useful in various applications such as in biaxial scanner or video display systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 09/138,367,filed Aug. 26, 1998, now U.S. Pat. No. 6,201,629 which claims benefit toU.S. Provisional application Ser. No. 60/057,700, filed Aug. 27, 1997.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support under Contract No.DAAK60-96-C-3018 awarded by the Soldier Systems Command of the UnitedStates Army. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Micro-electromechanical system (MEMS) mirrors (or micro-mirrors) havebeen evolving for approximately two decades as part of the drive towardintegration of optical and electronic systems, for a range of usesincluding miniature scanners, optical switches, and video displaysystems. These structures consist of movable mirrors fabricated bymicro-electronic processing techniques on wafer substrates (for examplesilicon, glass, or gallium arsenide). The torsional micro-mirrortypically comprises a mirror and spring assembly suspended over a cavityformed in or on a base. The mirrors are electrically conductive, as isat least one region behind the mirror, affixed to the stationary base,so that an electric field can be formed between the mirror and the base.This field is used to move the mirror with respect to the base. Analternative comprises the use of magnetic materials and magnetic fieldsto move the mirrors.

Typically the mirror surface consists of either the wafer itself or adeposited layer (metal, semiconductor, or insulator), and generally inthe prior art the springs and mirror are formed from the same material(but not in all cases). The mirror and torsion springs are separatedfrom the base by an etch process, resulting in the formation of a cavitybetween the mirror and base.

For display or image acquisition applications, the goal is to developcompact systems with rapid frame rates (at least 60 Hz) and highresolution, consisting of between 200 and 2000 miniature pixels perline. For scanning system designs in this range, the mirrors should belarge (in the range of 200 μm×200 μm to 2 mm×2 mm), fast (in the rangeof between 3 kHz and 60 kHz for resonant devices), and scan a photonbeam through a large angle 7 to 40 degrees).

In optical systems that contain very small elements, diffraction by thesmallest element may introduce diffraction broadening and deleteriouslyincrease the final pixel size. Enlarging the limiting element reducesthis broadening and militates for larger mirrors. However, as mechanicalsystems get larger (for example, increasing the size of a torsionalmirror), they tend to be characterized by greater mass and consequentlylower resonant frequency; this resonant frequency sets the scanningspeed of the system. A frequency in the range of 5 to 50 kHz isdesirable. Prior art mirror designs have been limited by the difficultyinherent in obtaining a high resonant frequency with a large mirrorsize, free from diffraction broadening effects. In prior art cases inwhich the mirror mass is made very low to obtain high resonantfrequency, the resultant reduction in stiffness of the mirror is alimiting factor in the quality of the reflected image. This problem isexacerbated by the possibility of heating of the mirror by lightabsorbed in the mirror. Such heating militates for a thick mirrorcapable of conducting the heat away from the source.

The scanning angle through which the mirror moves determines the numberof distinguishable pixels in a display or imaging system. Therefore, alarge scanning angle is desirable. Generally in the prior art the scanangle is limited by the presence of electrodes that interfere withmirror motion (but not in all cases).

Electrostatic actuation is the most common method used to drivemicro-mirrors. In order to produce a force, a voltage is generatedbetween two electrodes, usually the plates of a parallel platecapacitor, one of which is stationary and the other of which is attachedto the mirror as described previously. By making the mirror anelectrical conductor, the mirror itself can be made to serve as one ofthe plates. The force generated for a given voltage depends on the platearea and on the gap between the plates, which may change as the mirrorposition changes. For torsional mirrors, the important drive parameteris the torque, and the effective torque on the structure is alsoproportional to the distance between the resultant force and the axis ofrotation of the mirror. Thus, a large driving force can be achievedusing large capacitor plates and small gaps; by applying the force at adistance from the rotation axis, a large torque may be obtained.

In many prior art designs the criteria for a large deflection anglerange tend to be in conflict with the criteria for large driving forces.The deflection angle is limited by the presence of surfaces behind themirror. An example of a limiting surface would be the bottom of a cavityin the base etched beneath the mirror, or some other substrate on whichthe mirror is mounted. The maximum angle is achieved when the mirrorcontacts this backplane, so the small separation between the mirror andthe backplane needed for generating adequate electrostatic deflectionforce limits the maximum angle. Accordingly, in prior art designs inwhich the mirror is used as one of the drive electrodes and the otherelectrode is on the backplane, increasing the gap reduces the force ortorque obtained at a given voltage. Some prior art designs useelectrodes that are offset from the main mirror body and which areconnected through actuator linkages, allowing the backplane to be movedfurther away or even eliminated entirely. Typically, though, theseelectrodes have smaller active areas and shorter moment arms, which tendto reduce the effective forces and torques as well. Additionally, if asthe mirror moves, the gap between the drive electrodes narrows, then thegap still may be a limiting factor for the range of motion of thestructure.

A second set of design problems arises in the selection of the mirror.Prior art designs and processes do not permit the mirror to be made fromvery low mass material without also sacrificing structural rigidity. Oneof the process limitations is the use of the same material for torsionspring and mirror mass, or the same set of patterning steps for springand mirror mass. The selection of mirror materials with a view towardthe elastic or fatigue properties of the springs restricts thesuitability of the material with respect to mirror mass rigidity, andalso limits the optical performance of the mirrors.

In 1980, Peterson disclosed a silicon torsional micromachined mirror(U.S. Pat. No. 4,317,611; K. E. Peterson, “Silicon torsional scanningmirror,”) IBM J. Res. Dev., 24(5), 1980, pp. 631-637). Both the mirrorand torsion elements were patterned in a thin (134 microns) siliconwafer and retained the full thickness of the wafer. The structure wasthen bonded to a glass substrate, over a shallow well to allow room forthe mirror motion. Actuation of the device was electrostatic. The mirrorbody was used as one electrode and the other electrodes were placed atthe bottom of the well under the mirror. A narrow ridge in the wellunder the axis of rotation of the mirror was used to eliminatetransverse motion of the structure. The manufacturing process for thisdevice was relatively simple, requiring a single patterning step for thesilicon and two patterning steps for the glass substrate. Its resonancefrequency was about 15 kHz, and at resonance the angular displacementreached about 1°. The limitations of this device are related to thedepth of the well. A 2 mm mirror touches the bottom of a 12.5 μm well ata displacement of 0.7° (1.4° total motion). Increasing the well depth toincrease the range of motion is not necessarily desirable, because itproportionally reduces the torque achieved for a given voltage.

Nelson (U.S. Pat. No. 5,233,456), Baker et al (U.S. Pat. No. 5,567,334),Hornbeck (U.S. Pat. No. 5,552,924), and Tregilgas (U.S. Pat. Nos.5,583,688 and 5,600,383) have developed and patented a series oftorsional mirror designs and improvements for use in deformable mirrordevice (DMD) displays. These mirrors are fabricated by surfacemicromachining, consisting of a series of patterned layers supported byan undisturbed substrate. The DMD display uses an individual mirror ateach pixel. The mirrors are therefore designed to be very small, to beoperated in a bi-stable mode, and to maximize the packing fraction onthe surface of the display. To minimize the gaps between the reflectingsurfaces of adjacent mirrors, the support structure and drive componentsare fabricated in underlying layers, requiring a complicated multi-stepdeposition and patterning process. As with the Peterson mirror, theHornbeck mirror is designed to serve as one of the deflectionelectrodes, and the others are placed behind the mirror. Owing to thesmall size of the mirrors (about 20 μm×20 μm), high deflection anglesare attainable with reasonably small gaps. These mirrors are designedfor driving at low frequencies, and for significant dwell at a givenangle (on or off), rather than for continuous motion, although the earlydevelopment included mirrors designed for resonant operation (U.S. Pat.No. 5,233,456). A scanned display or imager requires, however, a largemirror, and the difficulties with scaling up torsional mirrors that aredriven electrostatically with plates mounted behind the mirrors preventthe Hornbeck mirrors from being easily modified for use in scanningdisplay applications.

Toshiyoshi describes a silicon torsion mirror for use as a fiber opticswitch (H. Toshiyoshi and H. Fujita, “Electrostatic micro torsionmirrors for an optical switch matrix,” J. MicroelectromechanicalSystems, 5(4), 1996, pp. 231-237). The Toshiyoshi mirror is a relativelylarge device (400 μm on a side and 30 μm thick), which rotates about anaxis close to one edge of the mirror. The mirror is defined by etchingthe silicon wafer from the front, and the excess wafer material isetched from the back of the wafer. It is thus suspended over a cavity inthe wafer, supported by very thin (0.3 μm) metal torsion rods. Thestructure is then bonded onto another substrate, on which electrodeshave been plated. Toshiyoshi has demonstrated separation of themechanical properties of the springs and mirror by using silicon for themirror mass, and metal for the springs. Actuation is electrostatic, byplacing a voltage between the mirror body and the electrodes of thelower substrate. The range of motion is limited by the mirror hittingthe glass substrate, at about 30°. In order to obtain the maximumdeflection at an applied voltage of 80 volts, the stiffness of thetorsion members must be very low, achieved by making them very thin.This also limits the resonant frequency of the structure to 75 Hz,making the approach unsuitable for a scanned display or scanned imager.Thus Toshiyoshi has not shown how the separation of the mechanicalproperties of the spring and mirror can be used to attain a highresonant frequency and high angular displacement.

Dhuler of the MCNC has disclosed a mirror wherein the mirror body isformed from the silicon substrate, while the supports and actuators arefabricated above the mirror plane using surface micromachinedpolycrystalline silicon layers (V. J. Dhuler, “A novel two axis actuatorfor high speed large angular rotation,” Conference Record of“Transducers '97, ” 1997). The mirror body is first defined using ionimplantation of boron as an etch stop, and then by removal of the excessSi wafer from the back of the mirror. The supports and drive electrodesare offset from the top surface of the substrate by posts, which definethe gap between the drive capacitor plates. Thus the mirror is free torotate unhindered by the bottom surface of a well, while the drivetorque, being applied by actuators, is not limited by a requirement fora large capacitor gap. While it represents a significant advance in thestate of the art, this device suffers from certain flaws which thecurrent invention resolves.

In the MCNC process the mirror body thickness is limited by the boronimplantation process, which has limited penetration depth; the disclosedmirror was 4 μm thick. The stiffness of the mirror is limited by bothits size and thickness, so larger mirrors need to be thicker to avoiddeformation of the mirror surface in use. For scanning applications,flexure in the mirror leads to uncertainty in the pixel size andlocation and distortion of the pixel shape. The implantation processalso introduces stress into the mirror body, causing deformation of thereflective surface. The supports and actuators of the MCNC device areformed in a multi-step process and, as they are non-conducting, requirethe separate deposition and patterning of electrodes.

Kiang describes a 200 μm×250 μm mirror that has a frequency of 15 kHzand maximum displacement of 150 (M. H. Kiang, “Surface micromachinedelectrostatic comb driven scanning micromirrors for barcodeapplications,” 9th Annual Workshop on Micro Electro-Mechanical Systems,1996, San Diego, Calif. pp. 192-197). This mirror is made of depositedand patterned surface layers, and before using it must be first rotatedout of the plane of the substrate using a comb drive and locked intoposition using complicated hinges. This approach obviates the problem offorming a cavity behind the mirror. However, the use of surfacemicromachined layers means that the structural rigidity of themicro-mirror cannot be controlled (because the thickness is limited tothin (a few microns) layers). The mirror motion is obtained byelectrostatic drive applied by an actuator linked to one edge of themirror. The motion of the mirror is restricted by the actuationmechanism.

Other torsional micromirrors are mentioned in the literature (M.Fischer, “Electrostatically deflectable polysilicon torsional mirrors.”Sensors and Actuators, 44(1), 1996, pp. 372-274; E. Mattsson, “Surfacemicromachined scanning mirrors,” 22 d European Solid State DeviceResearch Conference, Sep. 14-17, 1992, vol. 19, pp. 199-204). Most aresmall (less than 100 μm on a side) and have very small displacements,not suitable for scanning applications. The exceptions tend to becomplicated to fabricate or actuate and suffer from the sameshortcomings as the mirrors described above.

Magnetically actuated cantilevered MEMS mirrors have been disclosed byMiller et al. of the California Institute of Technology (R. Miller, G.Burr, Y. C. Tai and D. Psaltis, “A Magnetically Actuated MEMS ScanningMirror,” Proceedings of the SPIE, Miniaturized Systems With Micro-Opticsand Micromachining, vol. 2687, pp. 47-52, Jan. 1996; R. Miller and Y. C.Tai, “Micromachined electromagnetic scanning mirrors,” OpticalEngineering, vol. 36, no. 5, May 1997). Judy and Muller of theUniversity of California at Berkeley disclosed magnetically actuatedcantilevered structures which may be used to support mirrors (Jack W.Judy and Richard S. Muller, “Magnetic microactuation of polysiliconflexure structures,” Journal of Microelectromechanical Systems, 4(4),Dec. 1995, pp. 162-169). In both cases, the moving structures aresupported by cantilever beams along one edge. They are coated with amagnetic material, and upon the application of a magnetic field at anangle to the mirror surface, the mirror rotates in the direction of thefield, bending the cantilevers. Miller has also disclosed a similarmirror which uses a small coil fabricated on the moving structure toprovide it with magnetic moment. In Miller's mirror, the springs areformed out of the original silicon wafer, and in Judy's mirror thesprings are fabricated out of a polysilicon layer deposited for thepurpose. The conduction path for the magnetic coil device is provided bya separate NiFe contact.

SUMMARY OF THE INVENTION

The invention relates to micro-machined optical-electro-mechanicalsystems (MOEMS), and, more particularly, to resonant and non-resonanttorsional micro-mirrors and their method of fabrication.

The principal embodiment of the present invention comprises a mirrorassembly rotatably supported over a cavity in a substrate or base. Atorsional mirror support assembly is provided comprising torsionalsuspension springs and force pads attached to the springs and to thebase. Actuation of the mirror is achieved by torsionally driving thesprings via the force pads, thereby causing rotation of the mirrorassembly. The upper surface of the mirror assembly may be coplanar withthe surface of the base. For the case in which the micro-mirror isformed from a silicon wafer, both the base surface and the mirrorsurface may be formed from the original silicon wafer surface (coated bymetal) so that if a polished wafer is used, a high quality mirror iseasily formed. The mirror support structure is suspended above a cavityin the base by micromachined torsional springs. The mirror is separatedfrom the base by etching away the wafer material from between the mirrorsupport structure and the base. The mirror support structure is providedwith a low mass stiffener, and the springs are provided withelectrostatic deflection plates, so that the actuation force is applieddirectly to the springs.

In an alternative embodiment, magnetic actuation of the mirror assemblyis provided. The mirror assembly includes a magnetic material thereon toprovide a permanent or temporary magnetic moment. A magnetic actuatorassembly is operative in conjunction with the magnetic material on themirror to rotationally drive the mirror. The magnetic material can coverall or a portion of a surface of the mirror assembly. The magneticmaterial can be applied to a surface of the mirror assembly in a patternpreselected to improve the magnetic and mechanical performance of thesystem, such as to minimize moment of inertia and lowering of resonantfrequency. Alternatively, the magnetic material can comprise aconduction coil formed on a surface of the mirror assembly, whereby amagnetic moment is formed when current is established within theconduction coil. The magnetic material can be formed along an edge ofthe mirror assembly, with an electromagnet disposed out of the plane ofthe mirror assembly.

The advantages of this invention over the prior art lie in thesimplicity of manufacture, the size and performance of the mirrorattainable in this design, and the accessible range of motion. Themirror can be made nearly as large as the starting wafer substrate(however, the sizes contemplated for the preferred embodiment aretypically in the range of 50 μm×50 μm to 3 mm×3 mm).

The resonant frequency depends on the mirror size; for a 600 μm squaremirror, resonant frequencies of over 20 kHz have been demonstrated, andwith minor design changes, frequencies appropriate for scanning atfrequencies above 30 kHz may be achieved. In one embodiment, the drivemechanism is electrostatic. However, several aspects of the inventionlend themselves well to magnetic actuation. Because the mirror itself issupported over a cavity in the substrate, large angular displacement ofthe mirror and its supporting structure can be achieved whilemaintaining a small gap between the plates of the drive capacitor formedat the supporting springs. The fabrication of the mirror is relativelysimple. The thickness of the mirror is easily controlled and may beadjusted to tune the resonant frequency or change the stiffness of themirror. The surface of the mirror may be metallized for greaterreflectance, or shaped to give it optical power.

In the process disclosed here, the mirror support structure is formedfrom the wafer substrate. The excess substrate material (if any) isfirst removed from the back of the mirror support structure by patternedetching, thus defining its thickness, mass, stiffness and thermalconductivity, while the mirror surface geometry is defined by patternedetching from the front. Using the substrate material to form the mirrorsupport structure has many advantages. The wafers are in generalavailable highly polished and extremely flat, giving good specularreflections (for example, Si and GaAs wafers intended for integratedcircuit production are flat and specular). The reflectance of suchwafers can be easily made to exceed 90% by metallizing the surface, forexample with a thin layer of aluminum. Such a layer can be sufficientlythin (less than 0.5 μm) so as not to introduce undesirable topologicalfeatures to the mirror surface. This is an advantage of the currentinvention over mirrors formed by surface micromachining, for example byelectrodeposition of metal or CVD polysilicon, which are generally roughand so require a separate polishing step.

Silicon is a good choice for the substrate because the mechanicalproperties of single crystal silicon are nearly ideal for micro-mirrorapplications. Silicon is light, strong, and stiff, yielding rigidmirrors with low moments of inertia. The process disclosed here, appliedto silicon, can yield a wide range of mirror thicknesses, and evenallows for engineered structures that may be used for the constructionof stiffer yet lighter mirror supports. The fabrication process for thecurrent invention is relatively simple, requiring only a limited numberof steps and mask levels. The springs in the current invention areconducting and serve as the top electrode, eliminating one fabricationlayer. Finally, this invention uses an electrodeposited metal layerwhich makes possible magnetically actuated designs, by choosing amagnetic material (such as nickel or permalloy) for the metal.

Accordingly, the present invention relocates the driving force, eitherelectric or magnetic, to sites that do not interfere to the same degreewith mirror motion. Also, the present invention provides a suitablylarge mirror while maintaining a high resonant frequency (low mass),adequate stiffness, and adequate thermal conductivity. A mirror of thisinvention overcomes the problem of obtaining high mirror mass andstructural rigidity, while also attaining the desired elastic constantsin the springs. The mirror also overcomes the problem of attainingmirrors with the desired optical properties, including optical power.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a perspective illustration of a torsional micro-mirror systemof the present invention;

FIG. 2A is a plan view of a torsional micro-mirror system of the presentinvention;

FIG. 2B is a cross-sectional view of the system of FIG. 2A taken alongcenterline 7;

FIG. 3 is a scanning electron micrograph of a micro-mirror system of thepresent invention;

FIG. 4A illustrates a plan view of a further spring embodiment;

FIG. 4B is a cross-sectional side view of the spring embodiment of FIG.4A;

FIG. 5 is a scanning electron micrograph of a spring embodiment;

FIG. 6A is a plan view of a further embodiment of a torsionalmicro-mirror system of the present invention;

FIG. 6B is a side view of the system of FIG. 6A;

FIG. 7 is a plan view of a further embodiment of a torsionalmicro-mirror of the present invention;

FIG. 8A is a schematic side view of an embodiment of a torsionalmicro-mirror system incorporating a magnetic actuation assembly;

FIG. 8B is a schematic isometric view of a further embodiment of amagnetically actuated system;

FIG. 8C is a schematic isometric view of a further embodiment of amagnetically actuated system;

FIG. 8D is a schematic plan view of a further embodiment of amagnetically actuated system;

FIG. 8E is a schematic isometric view of a further embodiment of amagnetically actuated system;

FIG. 9 is a schematic plan view of a further embodiment of an actuatorassembly for a torsional micro-mirror system of the present invention;

FIG. 10A is a schematic plan view of a further actuator assembly;

FIG. 10B is an image of a torsional micro-mirror such as that in FIG.10;

FIG. 11 is a plan view of a further embodiment incorporatingcantilevered springs to actuate torsional motion;

FIG. 12 is a schematic plan view of a multi-axis torsional micro-mirroraccording to the present invention;

FIG. 13A is a schematic cross-sectional view of a further embodiment ofa multi-axis torsional micro-mirror having wire-bond wire jumpersaccording to the present invention;

FIG. 13B is a schematic plan view of a multi-axis micro-mirror of FIG.13A having integrally fabricated contact structures;

FIG. 13C is a schematic view of a multi-layered torsional springcontaining multiple electrical paths to the mirror;

FIG. 14A is a schematic cross-sectional view of a torsional micro-mirrorincorporating a damping material surrounding the springs;

FIG. 14B is a schematic isometric view of a torsional micro-mirrorincorporating damping material at several positions along the movingedge of the mirror;

FIG. 14C is a schematic cross-sectional view of a torsional micro-mirrorwith a damping coating on the springs;

FIG. 14D is a schematic cross-sectional view of a torsional micro-mirrorwith high damping layers within the springs;

FIG. 15A is an image of a biaxial micro-mirror with wire-bond wirejumpers and damping with vacuum grease;

FIG. 15B is an image of the biaxial micro-mirror of FIG. 15Aillustrating a magnetic foil laminated to the back to provide a magneticmoment;

FIG. 16 is a schematic isometric view of a cantilevered micro-mirroraccording to the present invention;

FIG. 17 is schematic cross-sectional view of a micro-mirrorincorporating optical sensing;

FIG. 18A is a schematic plan view of a torsional spring

FIG. 18B is a schematic plan view of a further embodiment of a torsionalspring;

FIG. 18C is a schematic plan view of a further embodiment of a torsionalspring;

FIG. 19 is a schematic plan view of a micro-mirror with taperedsupports;

FIG. 20 is a schematic plan view of a micro-mirror with springs havingnecked down regions;

FIGS. 21A-21G are schematic cross-sectional views illustrating afabrication process for a micro-mirror according to the presentinvention;

FIG. 22A is a schematic side view illustrating the step in thefabrication process of providing a mirror;

FIG. 22B is a schematic side view illustrating the step in thefabrication process of providing a mirror with an adhesive or otherattachment layer, support layer, and a reflecting layer;

FIG. 22C is a schematic side view illustrating the step in thefabrication process of providing a mirror with an adhesive or otherattachment layer and a support layer and a curved reflecting layer;

FIG. 22D is a schematic side view illustrating the step in thefabrication process of providing a stress compensating layer; and

FIGS. 23-23G are schematic side views illustrating the fabricationprocess of providing a backside patterning.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a perspective illustration of a torsional micro-mirrorsystem. Support posts 1 are mounted to a base 2. A mirror 3 and mirrorsupport structure 4 are provided between the posts and are suspended bytorsional springs 5. A cavity 6 formed in the base 2 below and aroundthe mirror support structure 4 is provided to facilitate the rotation ofthe mirror. The deeper the cavity 6, the greater the rotation angle ofthe mirror. In some embodiments of the invention, the cavity extendsentirely through the base so that the base does not limit the rotationat all. It should also be noted that although the mirror shown in FIG. 1is rectangular, the mirror support structure and mirror surface may beany practical shape including round, ovoid, or octagonal, and may beselected to reduce the mass of the mirror support structure, while stillyielding a satisfactory mirror area.

FIG. 2A shows a plan view of one preferred embodiment, and FIG. 2B showsthe cross section of the system shown in FIG. 2A, taken along centerline7. The invention consists of a mirror surface 3, parallel in this casewith the surface of base 2 and suspended above an opening 6 in the baseby two torsional springs 5. The springs 5 are collinear and aligned withthe mirror support structure 4 centerline 7, and are offset from thesubstrate and mirror surface by posts 1. The mirror surface 3 may beformed by vapor deposition of a metal such as A1 onto mirror supportstructure 4. Metallized regions 8 on the base surface, under the spring5 and offset laterally from the centerline 7 form one plate of theelectrostatic drive capacitor, and the spring 5 serves as the otherplate. Electrical contact to the drive capacitor plates is made bymicrofabricated conduction lines, and isolation between the conductorsand the base is achieved by an oxide layer 13 formed on the surface ofthe substrate. The posts 1 offsetting the springs 5 from the substratesurface set the size of the capacitor gap.

A preferred embodiment of the invention is designed to be operated atthe resonance frequency of the device, which depends on the geometry andthickness of the mirror and the shape and material of the supports, aswill be discussed later. In brief, necked down regions 9 reduce thespring constant of the springs 5 and reduce the driving force requiredfor actuation. The reduced spring constant also reduces the resonantfrequency. The extent of motion in response to a given drive voltage isdetermined by the stiffness of the springs 5, including necked downregions 9, and the size of the gap 10 between springs 5 and plates 8,the area of the plates 8, and the quality factor Q of the resonantstructure. The maximum displacement is limited by the angle thetorsional spring can twist through before the edge of the springcontacts the base. A plan view scanning electron micrograph of a mirrorsystem formed in this way is shown in FIG. 3.

There are several key advantages of this invention over the prior art.First, this invention uses electrostatic actuation, applied to thesprings 5. FIG. 4A and B illustrate one spring design, in which it canbe seen that electrostatic force pads 11, 12, placed on an oxide 13 uponbase 2 (not behind the mirror), are connected to pads 14 and 15. FIG. 4Bshows a cross sectional view along line 16, and FIG. 5 shows a scanningelectron micrograph of a spring system formed in this way. Spring 5,(metallic in this embodiment), is connected to pad 17. An electrostaticdriving force can be applied between the spring 5 and the fixed forceplate 12 by biasing pad 15 (in electrical contact with 12 through wiretrace 18) with respect to pad 17 (in contact with spring 5 through trace19), and similarly, a force can be applied between fixed force plate 11and spring 5 by biasing pad 14 with respect to pad 17. Alternatelybiasing the pads with a periodic potential at or near the resonantfrequency of the system will excite resonant motion of the mirror.Alternatively, the resonant motion may be excited by biasing only oneplate and in such a case the other plate may be used for sensing themotion of the mirror by measuring the capacitance changes as the gapchanges. For example, an AC bias may be applied between spring 5 andplate 11 at or near the resonant frequency to induce motion, and a DCbias voltage applied between plate 12 and spring 5. The changes incapacitance between spring 5 and plate 12 owing to the motion of thespring 5 may be sensed by monitoring the current at pad 12. On a givenmirror support structure, one or both of the springs 5 may be biased andor sensed.

Note that in the embodiments shown herein the force pads are not behindthe mirror, thus the mirror motion is not limited by the problem of themirror contacting the force pad. The spring itself may contact the base,which will ultimately limit the range of motion of the mirror; however,the spring and force pads may be designed to yield a much greater rangeof motion, as we will show in the various preferred embodiments.Additionally, the fixed force plates 11, 12 may be designed so as toprevent electrical contact between the spring 5 and the fixed plates 11,12, for example by coating the plates with a dielectric or by limitingthe width of the plates so that the distance from the centerline to theplate edge is less than the product of one half the spring width and thecosine of the maximum deflection angle.

For certain applications, it may be desirable to modify the design bybonding external mirrors to the mirror support structure in a separatestep. A mirror may be physically bonded to mirror support structure 4(FIG. 2). In a different design (FIG. 6) the spring 5 also serves as themirror support structure, and extends across the cavity 6. The mirror isbonded directly to this spring. These external mirrors 20 could be madeof materials that are not convenient to include in the fabricationprocess, or could be made of mirrors having optical power. The approachshown in FIG. 6 has the additional and desirable feature that the cavityin the base wafer may be formed by etching only from the front side ofthe wafer.

Another embodiment of the invention uses a different electrostaticcapacitor design for the drive, and is shown in FIG. 7. A pair ofelectrodes 21 is formed along the edges of the mirror parallel to therotation axis. A second pair of electrodes 22 is formed on the basealong the edges of the cavity nearest the mirror electrode. The forceresulting from an applied voltage is proportional to the capacitancebetween the electrodes, which is inversely proportional to the gapbetween them. In this configuration no gap is required between thetorsional springs 5 and the base 2, so the fabrication of the posts maybe eliminated if desired. The length of the freely rotating portion ofthe springs 5 may be controlled by enlarging the width of the cavity 6so as to increase the distance 23 between the mirror support structure 4and the base 2. The distance 23 may differ from the distance 24 betweenelectrodes 21 and 22.

A variation of the design involves magnetic actuation of the structure(FIGS. 8A though 8D), again without restricting the freedom of motion ofthe mirror. In this case, magnetic material 25 is applied to the mirrorsupport structure (FIG. 8A) partially or totally covering the structuresurface in order to give it a magnetic moment. The driving field isprovided by an external electromagnet 26 which exerts a torque on themagnetic structure, causing the mirror support to rotate.

The electromagnet may be placed at a sufficient distance from the mirrorto allow free rotation. However, if only limited motion is required, themagnetic coil may be placed closer to the mirror, thus reducing the coilcurrent necessary to achieve actuation.

The magnetic material may be patterned to improve the magnetic andmechanical performance of the device. Specifically (FIG. 8B) thematerial may be confined near the axis of rotation so as to minimize itsmoment of inertia and avoid lowering the resonant frequency, and may beshaped into one or several elongated structures 27 so that the preferredmagnetic axis is perpendicular to the rotation axis.

The magnetic material may either have a permanent magnetic moment ortemporarily acquire a moment upon the establishment of an externalmagnetic field, in which case the torque is due to the shape anisotropyof the magnetic structure. The electrodeposited layer comprising thesprings may be chosen to be of a magnetic material, for example (but notnecessarily) permalloy, in which case the magnetic structure may beformed in the same operation as the fabrication of the springs, orincorporated into the design of the springs themselves. Alternatively,two different materials may be used for the springs and magneticstructures, which may be formed in separate steps. Alternately, themagnetic material may be separately deposited or laminated onto thestructure, in which case the magnetic material need not be formed by oreven be compatible with standard fabrication processes, allowing the useof, for example, ceramic ferromagnets.

Another approach comprises patterning of a conduction coil 28 (FIG. 8C)onto the mirror support structure, which creates a magnetic moment whencurrent is established. If multiple turns are used, a bridging structure29 may be fabricated to connect the center of the coil to the electricleads. The magnetic moment established when current flows in the coilinteracts with the field of a small permanent magnet 30 to rotate themirror support structure to the desired angle.

Yet another approach to magnetic actuation uses a reluctance circuitapproach (FIG. 8D). One or more external electromagnets 31 are mountedeither slightly displaced above the mirror structure or at an angle toit. Upon the establishment of a current in the coil, the magneticmaterial 32 is pulled towards the center of the coil, forming part ofthe electromagnet core. In this case, additional magnetic material 33may be deposited on the base 2 for the attachment of the external coils31 and used to define the shape of the applied magnetic field. Theelectromagnet may be fabricated on the wafer using standard techniques.

A fifth approach to magnetic actuation uses a permanent magnet attachedto the mirror structure to provide a magnetic moment normal to themirror surface. See FIG. 8E. In this configuration a magnetic fieldapplied parallel to the mirror surface and orthogonal to the axis ofrotation exerts a torque on the permanent magnet causing the mirror torotate. The field could be produced, for example, by a pair ofelectromagnet coils placed on either side of the mirror along an axisorthogonal to the rotation axis, with the coil axis parallel to thesurface of the mirror.

Yet another embodiment of the invention suspends the mirror about anaxis of rotation 34 displaced from its centerline 7, as shown in FIG. 9,allowing greater linear displacement on one side of the axis of rotationthan the other. In this embodiment a large movable capacitor plate 35 isarranged to push on the springs 5. The large fixed plate 36 is biasedwith respect to plate 35, thereby generating a large force which istransferred to the springs. The force of plate 35 may also betransferred directly to the mirror by an arm 37 as shown in FIG. 10A,which may or may not be attached to the mirror. The drive of themechanical actuator may be electrostatic, magnetic, or piezoelectric. Animage of a scanning electron micrograph of such a device is shown inFIG. 10B.

Alternatively, one or both springs may be split into two elements 99along the axis of rotation 7 (FIG. 11) in order to provide additionalelectrical paths onto the mirror support assembly. In this embodiment,rotational compliance of the springs may be replaced by bending ortwisting of the support posts 100 at the mirror support platform. Thishas the further advantage that the rotational motion of the mirrorsupport platform can result from cantilevered bending of the springs. Atthe fixed base end, the springs may be made as wide as desired toincrease the electrostatic force. Applying a voltage between the springsand the capacitor plates on the fixed base, alternately on each side ofthe centerline 7, causes the springs to bend and push down on thesupport platform. By attaching the springs to the platform near the axisof rotation, large angular displacement can be achieved for very smallvertical motion. This embodiment improves on a device such as describedby Dhuler in several important aspects. First no bearing or additionalelement is required for supporting the moving platform, simplifying thefabrication and eliminating wear associated with surfaces moving againsteach other. Second, the springs and supports described in this inventionmay serve as electrical paths onto the mirror support platform.

Video images require sweeping in two orthogonal directions, but thesecond sweep direction need not move faster than the frame rate, rangingfrom 30-180 Hz. To obtain images, two separate mirrors could be used,rotating about orthogonal axes, or a single reflecting surface could bemade to scan both directions. A single mirror that scans two orthogonaldirections is achieved either by mounting the current invention on ascanning platform, or modifying the design so the reflecting surface issupported within a gimballed frame, and made to scan in both directions.The actuating mechanism for the two directions could be direct orindirect electric or magnetic force, or any combination thereof.

A multi-axial micro-mirror may be formed using the designs and processesdescribed herein. The actuation mechanism for the two directions isdirect or indirect electric or magnetic force, or any combinationthereof. FIG. 12 illustrates one possible multi-axial design: a firstpair of springs 38 is used for rotation of the mirror support structure4 along one axis, and a second pair of springs 39 is used for rotationalong a second axis which in this case is perpendicular to the firstaxis. The first pair 38 joins the base 40 to a movable support frame 41;this support frame 41 is connected to the mirror support structure 4 bythe second pair 39. (Other pairs and additional movable supports may beadded that can be designed to operate at resonance or in a bi-stablemode with the advantage of providing aiming or alignment of themicro-mirror system. Other pairs may also be useful in distortioncorrection.) The deflection voltage is supplied to the pair 38 throughbias applied to pads 42 and 43, and through bias applied through pads 44and 45. The deflection voltage for pair 39 is supplied through pair 38by traces 46 and 47. Thus, by relatively biasing the two springs in pair38, bias to the pair 39 may be attained, without addition of furtherconduction paths to the moving parts.

If separate electrical contacts are to be provided for the inner mirrorof a gimballed structure, they may be combined with the mechanicalsupport of the outer mirror support frame, or may be run throughseparate structures bridging the gap between the inner and outer supportstructures, as shown in FIG. 13. These separate structures may or maynot contribute to the mechanical properties of the system. Thestructures may either be added as part of the packaging process, such asthe addition of wire-bond wire jumpers 48 in FIG. 13A, or may befabricated along with the rest of the structure. An example ofintegrally fabricated contact structures which minimally impact themechanical properties is shown in FIG. 13B. Soft serpentine springs 49bridge the gap, and connect through traces 50 to the inner springs 39.The outer springs 38 are separately connected to the drive pads 11 and12 by traces 52 and 53 respectively. Multilayered spring supports mayalso be used to increase the number of separate electrical paths to theinner mirror. FIG. 13C shows such a spring; an insulating layer 54separates the upper conducting layer 55 from the lower conducting layer56. Each conducting layer connects to a separate pair of traces (notshown).

It may be desirable for the inner and outer support structures to havedifferent damping characteristics. For example, in a scanning display,the line scanning structure may be resonantly driven and benefit by lowdamping (high Q), while the frame scanning structure is driven linearlyat low frequency and benefits from high damping for uniform motion.Thus, the device can be operated in a vacuum package to minimize the airdamping of the fast mirror, with specific damping means provided for theslow scanning structure. For example, a damping material 57 may beapplied to the slow moving structure or the springs (FIG. 14A). Thisdamping material may consist of a liquid, gel, or soft semi-solidsurrounding the springs, with or without an enclosure to confine it.Many materials are suitable for this purpose, including vacuum greasesuch as Dow Corning DC 976 or Apiezon N type, RTV silicone, or spin-onpolymers used in the fabrication process such as polyimide orphotoresist (for example Shipley AZ1308). One embodiment is theapplication of a drop of DC 976 57 to the substrate so that it enclosesthe springs (FIG. 14A). Optionally, the damping material 98 may insteadbe applied along the moving edge away from the spring (FIG. 14B), sothat it bridges the gap between the moving support element and the fixedbase of the device. The damping material may be applied anywhere alongthe gap between the moving component and the fixed component, or betweentwo components moving at different velocities, for example, the mirrorplatform and the gimballed outer frame of a biaxial embodiment. Analternative is to coat the springs 38 with a high damping coating suchas photoresist 58 (FIG. 14C). Another alternative is to enclose a highdamping material within the springs, for example by making a multilayerstructure incorporating high strength layers 59 surrounding high dampinglayers 60. Damping may also be provided by mechanically attachingdamping devices (dashpots) between the moving structure and the base.

FIG. 15A is a micrograph image of a biaxial scanning display device. Theinner mirror support platform 4 is the line scanner, driven by anelectrostatic resonant drive. The oxide has been removed from itssurface and it has been coated with aluminum to form the mirrorreflective surface. The electrical ground lead contact is made throughthe outer frame supports 38 and the drive voltage contacts are made bywire-bond wire jumpers 48. The outer frame is the frame scanningdirection and is driven magnetically to give a slow linear sweep and afast retrace. Harmonic oscillations in the motion are damped by theapplication of approximately 0.005 microliters of DC 976 vacuum grease57 to dampen the slow axis springs 38. The magnetic moment is providedby magnetic foil 25 laminated to the back of the structure, coveringhalf the area and placed orthogonal to the axis of rotation (FIG. 15B).This device is then mounted on a small electromagnet and packaged invacuum.

In a preferred embodiment of a device as shown in FIG. 15, the mirrorsupport platform is approximately 1 mm×1 mm and has a resonant frequencyranging from 7 to 15 kHz. The resonant frequency of the outer frame is150 to 700 Hz. The outer frame is driven at 60 Hz. The device ispackaged in vacuum above an electromagnetic coil, preferably in a T05package containing optics.

In another embodiment of the invention, shown in FIG. 16, the mirrorsupport structure 92 is connected to the base 40 along one edge 94 byone or several metal cantilever springs 93, allowing the mirror supportstructure to rotate out of the plane of the wafer. The stiffness of thesupport springs 93 depends on the total aggregate width, but this widthmay be distributed among as many supports as desired (five in FIG. 16),each providing a separate electrical path onto the mirror supportstructure. Other devices, for example a torsional MEMS mirror such asdescribed herein, a CMOS circuit, for example the drive circuit, orboth, may be fabricated on or grafted onto the platform 92. The numberof possible electrical contacts is limited only by the length of theedge 94 and the minimum practical width and separation of the springs93. This embodiment is an improvement to the mirror disclosed in theliterature by Miller, in that the support springs are conducting and mayprovide multiple distinct electrical contacts to elements on the mirrorsupport platform. In addition, the platform in the present invention maybe formed of the original wafer, which may be a single crystalsemiconductor, and may have a desired thickness up to the full thicknessof the original wafer. Thus, it may comprise any desired CMOS circuit ordevice, which may be fabricated on the substrate prior to the release ofthe cantilevered platform.

The motion of the platform 92 may be used either to set the angle 95between the platform 92 (and any devices carried upon it) and the base40 (and any other devices attached to it), for example for the purposeof optical alignment, or to sweep the angle, for example for a display.The cantilever may be moved into place mechanically, for example duringthe fabrication or packaging of the device, and locked into place, tofix the angle. Alternatively, magnetic material may be applied to thefront, back, or both surfaces of the platform and a magnetic field maybe applied to it during operation to rotate it to the desired angle. Inthis case, a DC magnetic field bias may be applied to set the centerangle of motion, and an AC field superimposed on it to sweep the angle.Either the AC or DC component may be zero. In another variation,mechanical elements such as levers may be provided to set the angleduring operation.

Sensors may be added to the mirror to detect its position and the extentof the motion and provide feedback for the drive electronics. One sensordesign consists of capacitors similar to but separate from the drivepads, with detection of the current changing as a function of mirrorposition as previously described. If magnetic material is present on themoving structure, a magnetic sensor, for example a pickup coil, may beplaced in close proximity to detect the mirror position. Optical sensingmay also be utilized, as shown in FIG. 17. A light source 61 provides afocused beam 62 which is reflected off the mirror 3 and detected by adetector 63. As the mirror rotates, the intensity of light incident onthe detector 63 changes and can be correlated with mirror angle. Theangle of incidence 64 can be chosen so the sensing light does notinterfere with the display illumination, or an infrared source may beused. Alternatively, the detection may be made from the back of themirror structure by reflecting the light off the back of the mirrorsupport structure 4. Optionally, the mirror position may be determinedby sensing the light incident on it for display illumination. Detectionmay also be made by allowing invisible radiation such as infrared lightto pass through a specifically designed mirror surface coating to adetector mounted on the back of or underneath the device.

Support and Spring Design

For electrostatic actuation, the force is proportional to the square ofthe applied voltage and inversely proportional to the square of the gap12 (FIG. 2) between the drive capacitor plates. Making the gap smalleror the spring wider reduces the maximum angle of rotation but increasesthe applied torque, while making the gap 12 larger or the supportnarrower allows greater motion but reduces the torque resulting from agiven applied voltage. At the maximum displacement, at least someportion of the spring 5 touches the base 2. The present invention isdesigned so that the mirror itself is free to rotate to any degreewithin the elastic limits of the springs, and the displacement islimited by the rotation of the springs in relation to the base. For afixed gap height g, if at any given point z along the length of thespring its width is w(z), then the maximum rotation allowed at thatpoint is approximately

sin(φ)=2g/w(z).

The angle of rotation varies along the length of the spring, fromapproximately zero at the connection to the support post to the maximumat the mirror support structure, and the spring shape can be designed totake advantage of this, for example by making the spring wider near thefixed end and narrower at the mirror support structure, allowing agreater angle of rotation while still allowing for a large plate areaand thus allowing for application of a larger torque than would beobtained in a spring of uniform cross section.

Several possible spring designs are shown in FIG. 18. The stiffness ofeach design can be calculated from standard mechanical expressions, forexample as found in Mark's Handbook of Mechanical Engineering. T.Baumeister, editor in chief, Mark's Standard Handbook for MechanicalEngineers, 8th ed., McGraw-Hill Book Company, New York, 1978, Section 5.For the uniform cross section spring shown in FIG. 18A the torsionalstiffness K_(t) is given by $\begin{matrix}{K_{t} = {\frac{G}{3.5\quad l}\left( \frac{b^{3}h^{3}}{b^{2} + h^{2}} \right)}} & 1\end{matrix}$

where G is the shear modulus, l, b, and h are the length, width, anddepth of the member respectively, and the numerical constant depends onthe aspect ratio of the cross section. For non-uniform cross sectionsprings, the reciprocals of the stiffness of individual elements add togive the reciprocal total stiffness: $\begin{matrix}{K_{total}^{- 1} = {\sum\limits_{i}^{\quad}{K_{i}^{- 1}.}}} & 2\end{matrix}$

For example, introducing a necked down region, as in FIG. 18B, reducesthe stiffness to: $\begin{matrix}{{K_{total} = \frac{K_{1}K_{2}}{K_{1} + K_{2}}},{where}} & 3 \\{{K_{t} = {\frac{G}{3.5\quad l_{i}}\left( \frac{b_{i}^{3}h^{3}}{b_{i}^{2} + h^{2}} \right)}},} & 4\end{matrix}$

since the wide part 65 of the spring 5 and the necked down part 66 ofthe spring 5 have different lengths and widths, but the thickness of theelectrodeposited layer and the material properties are the same forboth. This type of design makes the device easier to actuate, but alsoreduces its resonance frequency. For a tapered support as shown in FIG.18C, the stiffness is given by: $\begin{matrix}{{K_{t} = {\frac{G\quad h^{3\quad}}{3.5\quad l}\frac{\left( {b_{\max} - b_{\min}} \right)}{\ln \quad \left( {b_{\max}/b_{\min}} \right)}}},} & 5\end{matrix}$

where the width varies from b_(max) at the substrate support post tob_(min), near the mirror.

For the case of a mirror support structure in which the rotation axiscoincides with the center of mass, the resonant frequency depends on themoment of inertia of the mirror element, J_(t), $\begin{matrix}{{J_{t} = \frac{\rho \quad L^{3}{Wt}}{12}},} & 6\end{matrix}$

where ρ is the density of the mirror support structure, L is its length,W is its width (parallel to the axis of rotation), and t its thickness.The resonance frequency is then: $\begin{matrix}{f = {\frac{1}{2\quad \pi}{\sqrt{\frac{K_{t}}{J_{t}}}.}}} & 7\end{matrix}$

For a given limiting angle and resonant frequency, the tapered design(FIG. 18C) allows the gap g to be reduced by a factor of:$\begin{matrix}{{\frac{g_{tapered}}{g_{uniform}} = \frac{\ln \quad \xi}{\xi - 1}},{where}} & 8 \\{\xi = \frac{b_{\max}}{b_{\min}}} & 9\end{matrix}$

over the gap for the uniform cross section design (FIG. 18A). For aspectratios ξ less than or approximately equal to 3, the limiting angle isimposed by contact between the spring and base at the narrow end of thespring. FIG. 19 illustrates a micro-mirror with tapered supports.

Another design is shown in FIG. 20, in which the spring 5 has two neckeddown regions 67, 68, which facilitate a high rotation angle. The neckeddown regions are separated by a wider region 69 which comprises themoving force plate 70 section of spring 5. The force plate 70 of spring5 is attracted alternately to plates 11 and 12 and can rotate through anangle φ given by the distance 67 (denoted w₆₇) and the gap (g), by theequation: sin(φ)=2g/w₆₇. The mirror support structure 4 can continue torotate through an additional angle which is limited by the elasticlimits of section 68 of spring 5. In general, for rotations of section67 less than φ and equal to α, the total rotation γ is given by:

γ=γw ₆₈ /w ₆₇.

By selecting the relative lengths of the necked down regions, largeangular displacements γ are possible with only a small movement α in theforce plate 70 integral to spring 5.

Fabrication

FIG. 21 shows the fabrication process. A polished wafer 71, preferablySi, is first coated on both sides with a material 72 on the front and 73on the back that is resistant to etches of the wafer material. Forsilicon, this material may be silicon nitride, silicon dioxide, or otherfilms known in the art. For the case of silicon dioxide to be formed onSi, the wafer may for example be oxidized to form a surface layer ofsilicon dioxide 72, 73 on both sides of the wafer, or the wafer may becoated by chemical vapor deposition, or by other means. Afterapplication of coating 72, 73, the wafer 71 and coatings 72,73 are thenpatterned on both sides with registered alignment marks and etched todefine the marks in the crystal. These marks, formed on both sides ofthe wafer, permit registration of features on the front and back(registration marks are not shown in FIG. 21).

Metal films, for example of chromium, gold, and titanium/tungsten alloy,are deposited on the front coated surface 72, and are patterned andetched to form pads 74 that provide the electrical contacts and anchorsfor the mechanical structures. The coating 73 is patterned and etched toact as a mask for wafer etching. The back of the wafer is then etched toform a membrane with surface 75 having thickness in the range of 20 μmto 200 μm. A typical thickness is 60 μm. The coating 72 on the frontsurface is then patterned and etched to form groove openings 76 in thecoating which will serve later in the process as an etch mask for theseparation of the mirror support structures 4 from the base 2. Theinitial coatings may also include or serve as the final mirror surface.

A release layer 77 of photoresist or other material is applied to thefront surface and patterned with holes 78 to expose the metal anchors74. After heat treatment, thin (0.05 μm to 0.5 μm) layers of a metal orsequence of metals such as chromium, gold and titanium/tungsten alloy 79are deposited on the front surface. Photoresist is then applied andpatterned to form a mask 80 for the electrodeposited structures. A metallayer 81, which may be nickel, is deposited by electroplating on to theexposed regions 82 of metal layers 74 and 79. The thickness of metallayer 81 is in the range of 0.5 μm to 10 μm; layer 81 constitutes thespring 5 in the plan views described earlier. The mask 80 and releaselayer 77 are removed by dissolving the layers in solvents orpreferential etches. This process also removes sections of intermediatemetal layers 83 (of metal layer 79) that are not reinforced by theelectroplating.

The wafer is diced, and the mirror support structure 4 is separated fromthe surrounding base 2 by etching both from the front, through thegrooves 76 defined in the etch masks 72 and 73, and from the back byetching surface 75, resulting in the formation of cavity 6 surroundingthe mirror support structure 4. The mirror support structure 4 is thusjoined to the base solely by the metal torsional springs 5. The finalthickness of the mirror support structure 4 depends on the duration ofthe two etch steps and can be selected to yield structures withthickness in the range of 10 to 200 μm or more (as large as the waferthickness if needed). Typical final support structure thickness is 30μm.

The final step comprises addition of a mirror, either by providing ametal or dielectric coating 84 (FIG. 22A) of the mirror supportstructure through a mask (for example with evaporated or sputteredaluminum with thickness in the range of 500 to 2500 angstrom), or bybonding a finished mirror to the support structure. FIG. 22B shows amirror bonded to the mirror support structure, comprising an adhesive orother attachment layer 85, a glass or other substance 86 as is used inthe art of mirror formation to support a reflecting layer, and a flatmirror (reflecting) layer 87. This invention additionally comprises thecapability to use a mirror comprising a curved surface formed inmaterial 86 and coated with a reflecting layer 88, as shown in FIG. 22C.This surface on material 86 may be either convex or concave, oraspherical. Additionally, the layers 87 or 88 may comprise a binaryoptical element of a diffractive or refractive nature, or a holographicelement.

Any of the coatings used in the fabrication process (for example thesilicon oxide layer or the optical coatings) may have a high degree ofinternal stress. If the mirror support structure is sufficiently thin,this stress could induce curvature in the structure. This curvature maybe useful, for example for shaping the optical surface, or may beundesirable. In the latter case, if it is not possible or desirable toremove the stressed film, additional layers 89 of equal stress may bedeposited on the back face of the structure to compensate for the stressand restore the surface flatness (FIG. 22D). Alternately, thecompensating layer may be applied to the front of the support structure,under the mirror, in which case the compensating layer stress would beequal and opposite to the stress already present. If the reflectance ofthe compensation material is sufficient (e.g. if chromium is used), itmay serve as the reflective surface as well.

The mirror thickness is determined by the duration of the etch used toremove the excess material from the back. The choice of thicknessresults from a tradeoff between mechanical stiffness, moment of inertia,and thermal conductivity. Thicker mirrors are stiffer, and so deformless in use, but the additional mass results in a higher moment ofinertia which lowers the resonant frequency. By adding backsidepatterning of the mirror support structure to the fabrication process asshown in FIG. 23, an engineered mirror support structure can be made, soas to yield a support which is stiff yet low in mass. For example, byadding a patterned mask 90 to the back etching, support ribs could befabricated 91 on the back of the mirror, perpendicular to the rotationaxis, adding stiffness but minimal weight. Alternately a corrugated,honeycomb, or other structure may be formed by etching a pattern ofwells into the back of the mirror support structure. Another method thatcan be applied when the substrate is a single crystal (such as Si) is topattern the back surface with an array of square openings aligned to thecrystal axis, followed by etching in a selective etch (such as potassiumhydroxide etching of Si) which etches {111} planes much more slowly thanother planes of the crystal. The etching process self terminates whenthe {111} planes are fully exposed, resulting in a pyramidal etch pit ofhighly controlled depth. In this way a corrugated structure with aprecise resultant mass is formed in the mirror support structure,without the need for a buried etch stop layer.

Structures having a magnetic moment, for use in magnetic actuation, maybe electroplated in the same manner as the springs. If the material usedfor the springs is magnetic, they may be deposited at the same time asthe springs. If two different materials are to be used, the magneticstructures may be electroplated in a separate step using a similarprocessing sequence to that described above. A third option for applyingmagnetic material to the structure consists of attaching a preformedmagnetic element to the completed device by means of an adhesive such ascyanoacrylate adhesive or Crystal Bond wax.

The invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims.

We claim:
 1. A process for fabricating a torsional micro-mechanicalmirror system, comprising: providing a wafer substrate of a substratematerial; providing electrical contact pads and anchors for torsionalspring structures on one surface of the wafer; forming the torsionalspring structures on the anchors; removing a portion of the substratematerial to define a mirror support structure separated by a gap fromsurrounding substrate material; and providing a mirror on the mirrorsupport structure.
 2. The process of claim 1, wherein the step offorming the torsional spring structures comprises: applying a releaselayer to the one surface of the wafer, the release layer patterned withholes to expose the anchors; depositing a layer of a conducting materialover the release layer to form a portion of the torsional springstructures; applying and patterning a photoresist layer to form a maskhaving exposed regions configured to allow deposition of material forthe torsional spring structures; depositing a material to the exposedregions to form the torsional spring structures; and removing therelease layer and mask.
 3. The process of claim 1, further comprisingetching a back surface of the wafer to form a membrane for the mirrorsupport structure.
 4. The process of claim 1, wherein the step ofproviding the mirror comprises providing a metal or dielectric coatingto the mirror support structure through a mask.
 5. The process of claim1, wherein the step of providing the mirror comprises bonding a mirrorto the mirror support structure.
 6. The process of claim 1, wherein thestep of providing the mirror comprises bonding a mirror base and areflecting layer to the mirror support structure.
 7. The process ofclaim 1, wherein the step of providing the mirror comprises bonding acurved mirror to the mirror support structure.
 8. The process of claim1, further comprising applying a stress compensation material to themirror support structure.
 9. The process of claim 1, further comprisingforming a stiffening pattern in a back side of the wafer.
 10. Theprocess of claim 1, wherein the step of providing a wafer comprisesproviding a polished wafer.